1. Technical Field
The present disclosure relates to a microelectromechanical (MEMS) z-axis detection structure, having low thermal drifts; in particular, the following treatment will make explicit reference, without this implying any loss in generality, to a microelectromechanical z-axis accelerometer.
2. Description of the Related Art
Z-axis inertial accelerometers of a MEMS type are known, including microelectromechanical structures that are sensitive to accelerations acting in a direction orthogonal to a plane of main extension thereof and to the top surface of a corresponding substrate (and that may also be able to detect further accelerations acting in the same plane).
FIGS. 1a and 1b show a MEMS structure of a known type, designated as a whole by the reference number 1, of a z-axis inertial accelerometer, which moreover comprises an electronic read interface (not illustrated), electrically coupled to the MEMS structure.
The MEMS structure 1 comprises a substrate 2 (for instance, made of semiconductor material, in particular silicon) having a top surface 2a, and a sensing mass 3, made of conductive material, for example polysilicon, and set above the substrate 2, suspended at a certain distance from its top surface 2a. The sensing mass 3 has a main extension in a sensor plane xy, defined by a first axis x and by a second axis y orthogonal to one another, and substantially parallel to the top surface 2a of the substrate 2 (in the condition of rest, i.e., in the absence of accelerations or any external stresses acting on the MEMS structure 1), and a substantially negligible dimension along an orthogonal axis z, which is perpendicular to the aforesaid sensor plane xy (and to the aforesaid top surface 2a of the substrate 2) and forms with the first and second axes x, y a set of cartesian axes xyz.
The sensing mass 3 has a through opening 4, which traverses it throughout its thickness, has in plan view a substantially rectangular shape extending in length along the first axis x, and is set at a certain distance from the centroid (or center of gravity) of the sensing mass 3; the through opening 4 consequently divides the sensing mass 3 into a first portion 3a and a second portion 3b, set on opposite sides with respect to the same through opening along the second axis y, the first portion 3a having a larger dimension along the second axis y as compared to the second portion 3b. 
The MEMS structure 1 further comprises a first fixed electrode 5a and a second fixed electrode 5b, which are made of conductive material, and are set on the top surface 2a of the substrate 2, on opposite sides with respect to the through opening 4 along the second axis y, so as to be positioned, respectively, underneath the first and second portions 3a, 3b of the sensing mass 3. The first and second fixed electrodes 5a, 5b have, in a plane parallel to the plane of the sensor xy, a substantially rectangular shape, elongated in the first direction x. The first and second fixed electrodes 5a, 5b hence define, together with the sensing mass 3, a first detection capacitor and a second detection capacitor with plane and parallel faces, designated by C1, C2, which have a given rest capacitance.
The sensing mass 3 is anchored to the substrate 2 by means of a central anchoring element 6, constituted by a pillar element extending within the through opening 4 starting from the top surface 2a of the substrate 2, centrally with respect to the same through opening 4. The central anchoring element 6 is consequently set equidistant from the fixed electrodes 5a, 5b along the second axis y, in a position corresponding to the center of gravity (or center of mass), designated by O, of the assembly formed by the fixed electrodes 5a, 5b. The center of gravity O is also used as the origin for the cartesian reference system xyz and corresponds to the single point of constraint of the sensing mass 3 to the substrate 2.
In particular, the sensing mass 3 is connected mechanically to the central anchoring element 6 by means of a first connection elastic element 8a and a second connection elastic element 8b, which extend within the through opening 4, with substantially rectilinear extension, aligned along an axis of rotation A parallel to the first axis x, on opposite sides with respect to the central anchoring element 6 and the center of gravity O. The connection elastic elements 8a, 8b are configured so as to be compliant to a torsion about their direction of extension, thus enabling rotation of the sensing mass 3 out of the sensor plane xy, about the axis of rotation A defined by the same connection elastic elements 8a, 8b. It is to be noted that the axis of rotation A passes through the center of gravity O and moreover constitutes an axis of symmetry for the central anchoring element 6 and the assembly of the fixed electrodes 5a, 5b. 
In use, in the presence of an acceleration acting in the orthogonal direction z, the sensing mass 3 turns, by the inertial effect, about the axis of rotation A, so as to approach one of the two fixed electrodes 5a, 5b (for instance, the first fixed electrode 5a) and to correspondingly move away from the other of the two fixed electrodes 5a, 5b (for example, from the second fixed electrode 5b), generating opposite capacitive variations of the detection capacitors C1, C2. A suitable interface electronics (not illustrated in FIGS. 1a, 1b) of the accelerometer, electrically coupled to the MEMS structure 1, receives at input the capacitive variations of the detection capacitors C1, C2, and processes them in a differential way so as to determine the value of the acceleration acting along the orthogonal axis z.